This invention relates in general to gas-fired heating apparatus and in particular relates to draft control arrangements for such apparatus.
A number of methods and devices have heretofore been suggested for improving the efficiency of gas-fired residential and commercial heating equipment. Certain of these devices are associated with the production and removal of flue gases and include automatic vent dampers, derating of heating equipment by throttling the gas input, fixed vent restrictors, and multi-stage or modulating burner controls.
Automatic vent dampers close the vent during standby periods. Such dampers reduce loss of heated room air through the vent and slow the cooling of the equipment, thereby saving heat for the next cycle which, in typical residential installations, begins five to ten minutes after the furnace is shut down by the room thermostat.
Derating can increase efficiency under certain circumstances but, by itself, unbalances the equipment vent system because the vent is too large for the reduced throughput of the derated equipment. This results in too much room air being aspirated through the relief opening of the draft hood, causing energy waste.
A fixed vent restrictor may be installed in the vent of a derated appliance. The vent restrictor reduces to a limited degree the excess flow in the vent. Since it is fixed and does not sense or compensate for changing draft or vent load situations it may cause spillage of flue gases at the relief opening of the draft hood under marginal draft conditions. During cold, windy weather its resistance to flow is insufficient to prevent excessive aspiration of heated room air.
Multi-stage or modulating control of heating equipment offers the advantage of adjusting the heat output of the heating equipment to actual demand at a particular time, thereby eliminating the wasteful on-off cycles. Also a considerable comfort improvement results from the continuous, even heat output. A heating appliance having multi-stage or modulating controls may be on full flame for ten to twenty minutes in the morning when the thermostat is turned up until comfort temperature is approached. The controls will then reduce the flame so that the predetermined comfort temperature is maintained.
At low flame the equipment-vent system is unbalanced similar to that described under "derating." The vent is too large for the heat throughput at low flame and causes excessive air flow with resultant heat loss. While a fixed vent restrictor can be adjusted to the requirements of a derated furnace in such a manner that the equipment-vent system works reasonably well under some conditions, this adjustment cannot be made for a furnace having multi-stage or modulating controls whose heat throughput may vary from full flame to low flame during the operating cycle.
Heaters having natural draft burners are equipped with draft hoods which prevent disturbance of the combustion process at the burner caused by excessive draft or backdraft in the vent. For economic reasons draft hoods in most domestic and commercial heating equipment are marginally designed, i.e., they tend to spill products of combustion during startup and under unfavorable weather conditions. The general trend to increase the efficiency of heating equipment means that in the future temperatures of the vent gases on the average will be lower than in the past. This means less buoyancy and therefore higher tendency for spillage at the draft hood. This tendency is accentuated by energy conservation devices in the vent, such as vent dampers and/or vent restrictors. Therefore the draft hood becomes a critical component of the equipment-vent system if the efficiency of the natural draft heating apparatus is increased and it is desirable to combine energy conservation devices and methods, such as vent dampers, vent restrictors and derating with more suitable draft hoods.
Another factor affecting the equipment-vent system efficiency is the sizing of vent connectors which are standardized in full inch increments. A four-inch vent connector has a 12.6 square inch cross section. A five-inch vent connector has a 19.6 square inch cross section, an approximately 55% increment. Since heating equipment is available in a wide variety of input ratings the vent connector available for a particular unit may be too small while the next size vent connector is oversized. This fact also calls for compensating means to make the equipment-vent system energy efficient.
It is desirable to combine heating equipment with a device that continuously adjusts the vent opening in proportion to the heat throughput of the equipment and to the draft situation. From an energy conservation standpoint the best device is one that reduces dilution air escaping through the draft hood to a practical minimum, regardless of heat output of the equipment or the weather condition at any given moment. A thermally controlled vent damper such as described in U.S. Pat. Nos. 3,228,605 and 3,510,059 can accomplish this task reasonably well. The wide variety of equipment designs, of equipment-vent combinations and of equipment and weather related variables and the accentuation of shortcomings of natural draft vent systems by the efforts to increase heating equipment efficiency make desirable vent control devices that retain the simplicity and reliability of the described thermally controlled vent dampers but which would be more adaptable to various designs of equipment and more efficient and precise in controlling heat loss through escape of room air.